1. Field of the Invention
The present invention relates to a sound absorbing structure comprising a porous member and a film having through holes laminated. In particular, it relates to a sound absorbing structure to be used for a soundproof cover.
2. Description of the Related Art
It is generally known that a porous member having communicating voids, such as a fibrous compact and an open cell foam material has a good sound absorbing characteristic. Therefore, it is used for the sound absorbing treatment for the inside of an engine cover or the inside of a bonnet of an automobile for the purpose of reduction of the noise from the automobile. However, according to the porous member, the sound absorbing material needs to be thick for improvement of the sound absorption coefficient in the middle or low sound range whereas in many cases a thick sound absorbing material cannot be installed due to the space limitation inside the engine cover or the bonnet. Therefore, a problem is involved in that a sufficient sound absorbing effect cannot be obtained by the sound absorbing material comprising the conventional porous member having the communicating voids.
Moreover, a foam material having a mixed cell structure of open cells and closed cells, and an open cell urethane foam with a film are also used as a sound absorbing material. However, although the foam material has a sound absorption peak at a relatively low frequency side, the peak value itself is not sufficiently high. Moreover, a thicker one has the peak shifted to the low frequency side, but since the frequency range of the peak itself is narrow, a sound absorbing effect may be obtained to some extent with respect to only a sound source with a specific single frequency or a frequency in the vicinity thereof by using a material with a thickness corresponding to the frequency. However, for example, in the case of the inside of an engine cover or the inside of a bonnet, due to the structure limitation, the foam material thickness cannot be changed freely in most cases. Moreover, since the automobile engine room noise in general has a frequency range to some extent, a sufficient sound absorbing effect cannot be obtained by the foam material having a mixed cell structure having a narrow sound absorption coefficient peak frequency range, with the peak frequency dependent on the thickness.
Moreover, a foam material having a cell structure with only closed cell is also used, but it has a low sound absorption coefficient in the entire frequency range so that it hardly provides the sound absorbing effect.
Furthermore, a perforated board as a resonant sound absorbing structure, comprising a hard board with through holes having an air layer on the back side is also used. Although an ordinary perforated board has a relatively high sound absorbing characteristic in a single frequency range, it shows only a low sound absorbing characteristic as a whole. It is known that the sound absorbing characteristic can be improved by disposing a urethane open cell foam or a glass wool in the perforated board back side air layer, but the sound absorbing characteristic is not sufficient.
For example, JP-A-9-13943 discloses a sound absorbing structure as a combination of a sound absorbing base material and a perforated cover material. JP-A-56-157347 discloses a sound absorbing structure as a combination of a foam material and a perforated film. JP-A-56-157346 discloses a sound absorbing structure as a combination of a porous material and a soft resin sheet provided with an air chamber. However, these sound absorbing structures show a high sound absorbing effect only in a specific frequency range. Therefore, a problem is involved in that although the noise can be reduced only when the frequency range of the noise actually shed and the frequency range whereat the sound absorbing effect can be provided coincide, the noise cannot be reduced in most cases. Moreover, the sound absorbing structure should be thick in order to improve the sound absorbing effect of these sound absorbing structures so that in the case a thick sound absorbing structure cannot be installed owing to the space limitation, the noise reduction effect can be further lowered. Particularly in the case of the sound absorbing structure disclosed in JP-A-9-13943, a problem arises in that the sound absorption coefficient on the low frequency side is low.
The invention has been achieved in view of the circumstances, and an object thereof is to provide a sound absorbing structure and a soundproof cover having a good sound absorbing characteristic in a wide frequency range, capable of further improving the sound absorbing characteristic in a desired frequency range according to the purpose.
As a result of the elaborate discussion of the present inventors, it was found out that the sound absorbing characteristic in a desired frequency range can be improved easily by providing a film on at least one side of a porous member having communicating voids, and further providing through holes in the film so that the sound absorbing characteristic thereof can be controlled optionally, and that a high sound absorbing characteristic can be provided in a wide frequency range by laminating the sound absorbing structures so that the same or more sound absorbing characteristic can be provided by a half or less thickness with respect to the conventional sound absorbing materials comprising a foam material or a fibrous compact. Moreover, it was found out that the sound absorption coefficient on the low frequency side can be improved in the case where at least one of the through holes has an opening area of 19 mm2 or more, and the total of the opening area of the through holes accounts for 1 to 70% with respect to the area of a film formation surface of the porous member. Furthermore, it was found out that a soundproof cover having the excellent noise insulation performance can be provided by mounting such a sound absorbing structure on a cover main body. The invention is based on the knowledge.
That is, in order to achieve the objects, the invention provides a sound absorbing structure comprising a film with through holes formed, laminated on a porous member having communicating voids at least on the side facing a sound source, wherein at least one of the through holes has an opening area of 19 mm2 or more, and the total of the opening area accounts for 1 to 70% with respect to the area of the film formation surface of the porous member (hereinafter referred to as a xe2x80x9cfirst sound absorbing structurexe2x80x9d).
Moreover, the invention provides a sound absorbing structure comprising a structure having a film without a hole laminated on at least one surface of a porous member having communicating voids as a lower layer, and the first sound absorbing structure as an upper layer, with both films laminated so as to face a sound source (hereinafter referred to as a xe2x80x9csecond sound absorbing structurexe2x80x9d).
Furthermore, the invention provides a sound absorbing structure comprising two or more layers of the first sound absorbing structures, with the film having the through holes of each sound absorbing structure facing a sound source, laminated such that the total of the opening area of the through holes is successively reduced with that of the sound absorbing structure disposed closest to the sound source maximum and that of the sound absorbing structure disposed farthest to the sound source minimum (hereinafter referred to as a xe2x80x9cthird sound absorbing structurexe2x80x9d).
Still further, the invention provides a soundproof cover comprising the first to third sound absorbing structures disposed on the inner surface of a cover main body.